1. Field of the Invention
The present invention relates to a resistor-capacitor (RC) oscillator in which a resonance frequency is determined by a resistor and a capacitor, and more particularly, to a self-calibrating RC oscillator which is less dependent on change in external temperature and can be integrated into one chip.
2. Description of the Related Art
In 2000, the Institute of Electrical and Electronics Engineers (IEEE) launched a move to standardize a low-speed wireless private network application into IEEE 802.15.4. This standardized version requires a battery to run for at least 5 to 7 years, which thus necessitates designing of a circuit consuming minimal power. The most widely known technology for minimizing power consumption is to turn off a circuit when it does not need to be in operation.
Meanwhile, in a system where a power is turned off during deactivation for less power consumption, the power should be turned on and off through a shorter time to reduce power consumption of the system. Especially, this characteristic is notably important in an oscillator circuit for generating a reference frequency which is essentially used in a telecommunication system.
In general, a conventional crystal oscillator needs time for self-start to amplify noises generated internally to an operational level. Accordingly, the conventional crystal oscillator entails a long operation time and significant power consumption to start oscillating. In consequence, the crystal oscillator with such operation time and power consumption is ill-suitable for a system which has a power turned off during deactivation to minimize power consumption.
To overcome problems with the conventional crystal oscillator, a resistor-capacitor (RC) oscillator has been suggested. The RC oscillator is inexpensive, low in power consumption, short in starting time and easily controllable in a frequency.
FIG. 1 is a block diagram illustrating a configuration of a conventional RC oscillator. As shown in FIG. 1, the conventional RC oscillator includes a resistor part R, a capacitor part C, a bias circuit 11, a lamp-hold circuit 12, a comparator 12, a divider 14, a charge pump 15 and an oscillator part 16.
In a brief explanation of the conventional RC oscillator, the bias circuit 11 generates a preset reference voltage VREF1, and a ramp current IRAMP to be proportional to the reference voltage VREF1 and inversely proportional to a resistance value of the resistor part R.
The ramp and hold circuit 12 charges the capacitor part C using the ramp current during a predetermined charging time. Here, the charging time is set by the divider 14 which divides a period of an oscillation signal outputted from the RC oscillator by a predetermined divide ratio. The charging time is proportional to the period of the oscillation signal of the RC oscillator. After the charging, the ramp and hold circuit 12 maintains a both-end voltage of the capacitor part C.
The comparator 13 compares the reference voltage with the both-end voltage of the capacitor part C and outputs a comparison result. The divider 14 controls the charge pump 15 to increase or decrease a control voltage of the oscillator part 16. That is, in a case where the both-end voltage of the capacitor part C is greater than the reference voltage based on the comparison result, the comparator 13 increases a voltage VCP supplied to the oscillator part 16 by the charge pump 15, thereby increasing a frequency of the oscillation signal outputted from the oscillator part 16. In contrast, in a case where the both-end voltage of the capacitor part C is smaller than the reference voltage based on the comparison result, the comparator 13 decreases the voltage VCP supplied to the oscillator part 16 by the charge pump 15, thereby decreasing the frequency of the oscillation signal outputted from the oscillator part 16.
In this conventional RC oscillator, the oscillation signal has the frequency determined by the resistance value of the resistor part R and a capacitance value of the capacitor part C. Therefore, in order to obtain the oscillation signal with a desired frequency, it is of great importance to manufacture the RC oscillator in which the resistance value and capacitance value have a predetermined value in designing a circuit.
However, the resistor part R and the capacitor part C, when configured as a one-chip by a complementary metal-oxide semiconductor (CMOS) process, have the resistance value and capacitance value widely varied according to temperature change and process variation. For example, an RC time constant may be varied by about ±50%. In consequence, conventionally, the resistor and the capacitor are hardly integrated with other circuits into one chip. Yet, an external lumped resistor and capacitor having adequately selected values respectively are connected to the circuit ‘A’ configured as a one-chip.